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Inmetallurgy,refining consists of purifying an impure metal. It is to be distinguished from other processes such assmelting andcalcining in that those two involve a chemical change to the raw material, whereas in refining the final material is chemically identical to the raw material. Refining thus increases the purity of the raw material via processing.[clarification needed] There are many processes includingpyrometallurgical andhydrometallurgical techniques.
One ancient process for extracting thesilver fromlead wascupellation. This process involved melting impure lead samples in a cupel, a small porous container designed for purification that would aid in the oxidation process, while being able to withstand the heat needed to melt these metals in a furnace. This reaction wouldoxidize the lead tolitharge, along with any other impurities present, whereas the silver would not get oxidized.[1]
In the 18th century, the process was carried on using a kind ofreverberatory furnace, but differing from the usual kind in that air was blown over the surface of the molten lead frombellows or (in the 19th century) blowing cylinders.[2]
The Pattinson process waspatented by its inventor,Hugh Lee Pattinson, in 1833 who described it as, "An improved method for separating silver from lead"[citation needed]. It exploited the fact that in molten lead (containing traces of silver), the first metal to solidify out of the liquid is lead, leaving the remaining liquid richer in silver. Pattinson's equipment consisted a row of up to 13 iron pots, each heated from below. Some lead, naturally containing a small percentage of silver, was loaded into the central pot and melted. This was then allowed to cool. As the lead solidified it is removed using large, perforated iron ladles and moved to the next pot in one direction, and the remaining metal which was now richer in silver was then transferred to the next pot in the opposite direction. The process was repeated from one pot to the next, the lead accumulating in the pot at one end and metal enriched in silver in the pot at the other.[3][4] The level of enrichment possible is limited by the lead-silvereutectic and typically the process stopped around 600 to 700 ounces per ton (approx. 2%), so further separation is carried out by cupellation.[5]
The process was economic for lead containing at least 250 grams of silver per ton.[2]
TheParkes process, patented in 1850 byAlexander Parkes, uses moltenzinc. Zinc is notmiscible with lead and when the two molten metals are mixed, the zinc separates and floats to the top with ~2% lead. However, silver dissolves more easily in zinc, so the upper layer of zinc carries a significant portion of the silver. The melt is then cooled until the zinc solidifies and thedross is skimmed off. The silver is then recovered byvolatilizing the zinc.[2] The Parkes process largely replaced the Pattinson process, except where the lead contained insufficient silver. In such a case, the Pattinson process provided a method to enrich it in silver to about 40 to 60 ounces per ton, at which concentration it could be treated using the Parkes process.[6]
The initial product ofcoppersmelting was impure"blister" copper, which contained sulfur and oxygen. To remove these impurities, the blister copper was repeatedly melted and solidified, undergoing a cycle of oxidation and reduction.[7] In one of the previous melting stages, lead was added. Gold and silver preferentially dissolved in this, thus providing a means of recovering these precious metals. To produce purer copper suitable for making copper plates orhollow-ware, further melting processes were undertaken, using charcoal as fuel. The repeated application of such fire-refining processes was capable of producing copper that was 98.5-99.5% pure.[citation needed]
The purest copper is obtained by anelectrolytic process, undertaken using a slab of impure copper as theanode and a thin sheet of pure copper as thecathode. Theelectrolyte is an acidic solution of copper (II) sulfate. By passingelectricity through the cell, copper is dissolved from the anode and deposited on the cathode. However, impurities either remain in solution or collect as an insoluble sludge. This process only became possible following the invention of thedynamo; it was first used in SouthWales in 1869.[citation needed]
The product of theblast furnace ispig iron, which contains 4–5%carbon and usually somesilicon. To produce a forgeable product, a further process was needed (usually described asfining, rather thanrefining). From the 16th century, this was undertaken in afinery forge. At the end of the 18th century, this began to be replaced bypuddling (in apuddling furnace), which was in turn gradually superseded by the production of mild steel by theBessemer process.[8]
The term refining is used in a narrower context.Henry Cort's originalpuddling process only worked where the raw material was whitecast iron, rather than the greypig iron that was the usual raw material for finery forges. To use greypig iron, a preliminary refining process was necessary to remove silicon. The pig iron was melted in arunning out furnace and then run out into a trough. This process oxidized the silicon to form a slag, which floated on the iron and was removed by lowering a dam at the end of the trough. The product of this process was a white metal, known asfiners metal orrefined iron.
Precious metal refining is the separation ofprecious metals fromnoble-metalliferous materials. Examples of these materials include usedcatalysts,electronic assemblies,ores, or metalalloys.
In order to isolate noble-metalliferous materials,pyrolysis and/orhydrolysis procedures are used. In pyrolysis, the noble-metalliferous products are released from the other materials by solidifying in a melt to becomecinder and then poured off oroxidized. In hydrolysis, the noble-metalliferous products are dissolved either inaqua regia (consisting ofhydrochloric acid andnitric acid) or in a hydrochloric acid andchlorine gas in solution. Subsequently, certain metals can be precipitated orreduced directly with a salt, gas, organic, and/ornitro hydrate connection. Afterwards, they go through cleaning stages or arerecrystallized. The precious metals are separated from the metalsalt bycalcination. The noble-metalliferous materials are hydrolyzed first and thermally prepared (pyrolyzed) thereafter. The processes are better yielding when using catalysts that may sometimes contain precious metals themselves. When using catalysts, therecycling product is removed in each case and driven several times through the cycle.[citation needed]
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